1. Field of Invention
The present invention relates to a device and method for the detection and measurement of electrochemically active compounds, especially pollutants, such as SO.sub.2, H.sub.2 S, NO.sub.x, and hydrazines. More particularly, the device is for detection of these pollutants in the presence of high concentrations of other pollutants and at lower levels than previously capable of detection.
2. Discussion of Prior Art
In recent times a greater awareness has developed regarding the danger of human exposure to a wide variety of chemicals found in urban, suburban, and industrialized areas. Not only are high doses of toxic compounds hazardous (perhaps lethal) but adverse health effects are found to be caused by low level exposure over a long time period. Such toxic compounds are used in industry and in the home, and it is recognized that pollutant emission sources exist virtually everywhere.
Because of the ubiquitous nature of pollution and the deleterious health effects it can produce, there arises a need to both monitor and control pollutant emissions and human exposure to same. This invention is directed toward measurement of toxic gases and some of these electrochemically active compounds which are carcinogenic in nature.
Sensitive instrumentation is necessary to measure low levels of SO.sub.2, H.sub.2 S, NO, NO.sub.2, hydrazines, and the like so that the safety and health of the worker and the population in general, can be properly protected. A problem encountered in the development of such equipment is the difficulty experienced in the detection of low concentrations of the toxic gas being sensed or monitored, in the presence of high concentrations of CO and other interfering gases which are frequently present. A further problem is encountered when the instrument zero drift and noise is large and the signal being measured is small, thus limiting the useful lower detectable limit of such devices. Hence, although the electrochemical activity is known for a variety of pollutants, development of highly sensitive and highly selective instrumentation has been hindered.
One approach taken to improve the selectivity of the electrochemical sensors for these gases in the presence of CO has been to use a gold catalyst for the sensing electrode as described, for instance, in U.S. Pat. No. 3,776,832 to Oswin et al. This approach, however, has only been partially successful. For example, typical discrimination ratios for NO.sub.2 and H.sub.2 S in the presence of CO are -1000/1 and 2000/1, respectively. (The negative signal for the NO.sub.2 /CO ratio indicates that NO.sub.2 is electro-reduced whereas CO is electro-oxidized under the preferred conditions for device operation). Therefore, 1000 ppm CO will give a signal equivalent to minus 1 ppm NO.sub.2 (negative deflection on instrument meter), and 2000 ppm CO will give a signal equivalent to 1 ppm H.sub.2 S. Ten ppm CO will give a reading equivalent to 10 ppb NO.sub.2 (a 50% error in NO.sub.2 signal of a typical ambient) and a reading equivalent to 5 ppb H.sub.2 S (a 100% error in H.sub.2 S signal in a typical ambient). The magnitude of these percentage errors clearly points out the shortcomings of electrochemical instrumentation employing gold working electrodes in the detection of these pollutant gases in the presence of CO at usual ambient levels.
Similarly, the use of the carbon-supported gold catalyst as described in U.S. Pat. No. 4,042,464 (Blurton & Sedlak) has reduced the CO signals such that discrimination ratios are -10,000/1 and 20,000/1 for NO.sub.2 and H.sub.2 S, respectively. This is an improvement but yet accuracy is limited at the lower levels.
In addition, both of these prior art systems also possess a limitation as to their ultimate usefulness since when measuring very low concentrations of noxious gases the background current tends to drift. Background fluctuations are typically in the range of .+-.10 ppb over the course of several hours making continuous zero adjustment necessary for accurate measurements as well as causing appreciable instrument instability when measuring low levels continuously.